We’ve covered building the body of a floating robot and its drive mechanism. So this month,
we’ll wrap up the HoverBot with an explanation of its drive electronics. After that, you’ll have
all the information you need to build your own HoverBot. How you ultimately control your
HoverBot I will leave up to you.

46 SERVO 11.2010

The ducted fans of the HoverBot spin quite fast. So, rather than risk damaging their motors by slamming them from clockwise to counter-clockwise, I opted for a slower approach to controlling the fans: relays. The fans draw nearly two amps of current
and using a traditional transistor switch would mean losing
0.7 volts of the fan’s voltage and dumping 1.4 watts of
heat through the transistor. (I’m also trying to keep the
HoverBot light and don’t want to add an additional AAA
cell to make up for the voltage drop.) Therefore, the
HoverBot uses DPDT relays instead of transistors or
MOSFETs. The relays have a must-latch voltage of five volts
but a coil resistance lower than I like (they require the robot
controller to source more current than I’m comfortable
with). To get around the high current requirements of the
relays, I used a 2N3904 transistor to source the current for
each relay. The current required to operate the transistor is
so small there is very little wasted power (about 210 m W)
switching a relay on and off with it. The HoverBot uses five
relays in two circuits to control the three ducted fans in the
HoverBot.